Synthesis and Antiviral Activities of Cyanoacrylate Derivatives

It was found that title compounds 8d and 8e had the same inactivation effect against ... microscope (Beijing Tech Instrument Co., China) and were not ...
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J. Agric. Food Chem. 2008, 56, 5242–5246

Synthesis and Antiviral Activities of Cyanoacrylate Derivatives Containing an r-Aminophosphonate Moiety NING LONG, XUE-JIAN CAI, BAO-AN SONG,* SONG YANG, ZHUO CHEN, PINAKI S. BHADURY, DE-YU HU, LIN-HONG JIN, AND WEI XUE Center for Research and Development of Fine Chemicals, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, Guiyang 550025, People’s Republic of China

Target compounds 8 were obtained by the reaction of alkyl 2-cyano-3,3-dimethylthioacrylate or cyarylamide (7a-7e) and R-aminobenzylphosphonate (5a-5e) under reflux condition using ethanol as solvent. Their structures were clearly verified by spectroscopic data (IR and 1H, 13C, and 31P NMR) and elemental analysis. These compounds were shown to be antivirally active in the bioassay. It was found that title compounds 8d and 8e had the same inactivation effect against tobacco mosaic virus (EC50 ) 55.5 and 55.3 µg/mL) as the commercial product ningnanmycin (EC50 ) 50.9 µg/mL). To the best of our knowledge, this is the first report on the synthesis and antiviral activity of cyanoacrylate derivatives containing an R-aminophosphonate moiety. KEYWORDS: Cyanoacrylate; r-aminophosphonate moiety; antiviral activity; synthesis

INTRODUCTION

2-Cyanoacrylates, a class of highly potent herbicial compounds, are known to disrupt photosynthetic electron transportation at a common binding domain on the 32 kDa polypeptide of the photosystem II (PSII) reaction center (1, 2). Due to their versatile biological activities and promising application in agrochemistry, a large number of cyanoacrylate derivatives have been reported to show broad spectrum bioactivities, capable of acting as herbicides, insecticides, fungicides, plant virucides, and antitumor agents (3–8). In our previous work, we designed and synthesized some chiral cyanoacrylates with antiviral activity by replacing the methylthio moiety of some 2-cyano3-methyl-thio-3-substituted-phenylacrylates with (R)- or (S)-1phenylethylamine groups. The (E)-configuration of the reported chiral products was confirmed by X-ray single-crystal structure analysis. The bioassays showed that a chiral compound containing a 4-nitrophenyl moiety [(E)-ethyl 3[(S)-1-phenylethylamino]3-(4-nitrophenylamino)-2-cyanoacrylate] exhibited good protection activity against tobacco mosaic virus (TMV) in vivo (9). Some alkyl 2-cyano-3-methylthio-3-phosphonylacrylates were found to possess good in vivo curative, protection, and inactivation effects against TMV with inhibitory rates at 500 mg/L (10). Recently we reported the synthesis and antiviral activity of (E)-ethyl 3-(R)- or (S)-1-phenylethylamino-3(substituted-phenylamino)-2-cyanoacrylate. The objective of our study was to show the effects of (R)-4p in inducing disease resistance of tobacco leaves in response to virus pathogen attack and on the activity of enzymes or metabolites that might be

involved in induced resistance (11). On the other hand, R-aminophosphonic acids, bioisosteres of natural amino acids, have been found to exhibit a wide range of bioactivities. Some derivatives of R-aminophosphonic acids find application as plant growth regulators, fungicide, plant virucide, herbicides, and so on (12–14). A large volume of research on their synthesis and biological activities has been reported during the last 10 years (15–20). In our previous work, while many substituted aryl aminophosphonate derivatives were shown to have good antiviral activities (21–26), and carbonylaminophosphonates possessed better antiviral activities (27). As different aminophosphonates and their derivatives display potential bioactivities, screening of cyanoacrylate derivatives bearing various R-amiophosphonate moieties might produce new lead compounds with more potent antiviral activities against TMV. Keeping these considerations in mind, we herein designed and synthesized some novel cyanoacrylate derivatives containing R-aminophosphonates (Scheme 1). The synthetic route is shown in Scheme 2. The bioassay test showed that the new compounds 8 possessed moderate to good antiviral activities. To the best of our knowledge, this is the first report on the synthesis and antiviral Scheme 1. Structural Features of Aminophosphonates versus Cyanoacrylate Analogues Containing Aminophosphonates

* Corresponding author. E-mail: [email protected].

10.1021/jf800405m CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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Scheme 2. Synthetic Route to Cyanoacrylate Analogues 8 Containing R-Aminophosphonate

activity of cyanoacrylate derivatives containing an R-aminophosphonate moiety. MATERIALS AND METHODS Instruments. The melting points of the products were determined on a XT-4 binocular microscope (Beijing Tech Instrument Co., China) and were not corrected. The IR spectra were recorded on a Bruker VECTOR 22 spectrometer with KBr disk. 1H, 13C, and 31P NMR (solvent DMSO-d6 and CDCl3) spectra were performed on a JEOLECX 500 NMR spectrometer at room temperature using TMS as AN internal standard. Elemental analysis was performed on an Elementar Vario-III CHN analyzer. Analytical TLC was performed on silica gel GF254. Column chromatographic purification was carried out using silica gel. All reagents were of analytical reagent grade or chemically pure. All solvents were dried, deoxygenated, and redistilled before use. Synthetic Procedures. Diethyl phosphite and di-n-propyl phosphite were prepared according to literature method as described (28). Intermediates 5a-f, 6a-e, and 7a-e were prepared according to the reported methods (29) and a detailed procedure can be found in the Supporting Information. General Procedure for the Preparation of Title Compounds 8a-8x. A mixture of alkyl (imido) 2-cyano-3,3-dimethylthioacrylate (7a-e) (0.1 mmol) and R-aminobenzylphosphonate (5a-e) (0.1 mmol) in ethanol (10 mL) was refluxed for 4 h. Upon completion of the reaction, the solvent was removed under reduced pressure and the residue was washed with water, filtered off, and purified by silica gel column chromatography (petroleum ether-ethyl acetate, 2:1, v:v) to give the title compounds 8a-x in 25.6-70.0% yields. The reperesentative data for 8a is shown below, while data for 8b-8x can be found in the Supporting Information. Data for Methyl 2-Cyano-3-methylthio-3-(diethyl aminobenzylphosphonyl)acrylate (8a). Colorless liquid; 51.3% yield. IR (KBr):

νmax 3062.9, 3030.1, 2983.8, 2953.0, 2208.4, 1643.3, 1566.2, 1556.5, 1514.1, 1494.8, 1440.8, 1384.8, 1257.5, 1161.1, 1045.4, 1020.3, 975.9, 840.9, 781.1, 748.3, 698.2, 561.2. 1H NMR (500 MHz, DMSO-d6): δ 1.28-1.29 (m, 3H, C-CH3), 1.31-1.32 (m, 3H, C-CH3), 2.54 (s, 3H, S-CH3), 3.82 (s, 3H, O-CH3), 4.05-4.91 (m, 2H, O-CH), 5.59-5.64 (m, 1H, Ar-CH), 7.34-7.40 (m, 5H, Ar-H), 10.94 (s, 1H, C-NH). 13 C NMR (CDCl3, 125 MHz): δ 172.26, 172.18, 171.18, 168.45, 129.08, 128.76, 127.66, 127.63, 117.93, 63.89, 58.16, 56.94, 52.22, 18.49, 16.47, 16.43, 16.37. 31P NMR (CDCl3, 500 MHz) δ: 19.21. Anal. Calcd for C19H27N2O5SP (440.15): C, 51.25; H, 5.82; N, 7.03. Found: C, 51.01; H, 5.90; N, 6.81. X-ray Diffraction. Colorless blocks of 8v (0.29 mm × 0.24 mm × 0.21 mm) were counted on a quartz fiber with protection oil. Cell dimensions and intensities were measured at 273 K on a Bruker SMART CCD area detector diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å), θmax ) 25.00, 19 767 measured reflections, and 3232 independent reflections (Rint ) 0.1763) of which 4767 had I > 2δ(I). Data were corrected for Lorentz and polarization effects and for absorption (Tmin ) 0.7680, Tmax ) 0.8237). The structure was solved by direct methods using SHELXS-97; all other calculations were performed with Bruker SAINT System and Bruker SMART programs. Full-matrix least-squares refinement based on F 2 using the weight of 1/[σ2(Fo2) + (0.0703P)2 + 0.0000P] gave final values of R ) 0.0558, ωR ) 0.1610, and GOF(F) ) 1.052 for 307 variables and 4767 contributing reflections. The maximum shift/error ) 0.001, and max/min residual electron density ) 0.400/-0.322 e Å-3. Hydrogen atoms were observed and refined with a fixed value of their isotropic displacement parameter. Antiviral Biological Assay. Purification of Tobacco Mosaic Virus. Using Gooding’s method (30), the upper leaves of Nicotiana tabacum L inoculated with TMV were selected, ground in phosphate buffer, and then filtered through a double-layer pledget. The filtrate was centrifuged at 10 000g, treated twice with PEG, and centrifuged again.

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The whole experiment was carried out at 4 °C. Absorbance values were estimated at 260 nm using an ultraviolet spectrophotometer. 0.1%,260nm virus concn ) (A260 × dilution ratio) / E1cm

(1)

Protective Effects of Compounds against TMV in Vivo. The compound solution was smeared on the left side while the solvent served as the control on the right side of growing N. tabacum L. leaves of the same ages. The leaves were then inoculated with the virus after 12 h. A brush was dipped in tobacco mosaic virus of 6 × 10-3 mg/mL to inoculate the leaves, which were previously scattered with silicon carbide. The leaves were then washed with water and rubbed softly along the nervature once or twice. The local lesion numbers appearing 3-4 days after inoculation were counted (9). Three repetitions were conducted for each compound. Inactivation Effect of Compounds against TMV in Vivo. The virus was inhibited by mixing with the compound solution at the same volume for 30 min. The mixture was then inoculated on the left side of the leaves of N. tabacum L., while the right side of the leaves was inoculated with the mixture of solvent and the virus for control. The local lesion numbers were recorded 3-4 days after inoculation (9). Three repetitions were conducted for each compound. Curative Effect of Compounds against TMV in Vivo. Growing leaves of N. tabacum. L of the same ages were selected. The tobacco mosaic virus (concentration of 6 × 10-3 mg/mL) was dipped and inoculated on the whole leaves. Then the leaves were washed with water and dried. The compound solution was smeared on the left side and the solvent was smeared on the right side for control. The local lesion numbers were then counted and recorded 3-4 days after inoculation (9). For each compound, three repetitions were measured. The inhibition rate of the compound was then calculated according to the following formula (av means average, and controls were not treated with compound).

inhibn rate (%) ) av local lesion no. of control av local lesion no. of drug-treated × 100% (2) av local lesion no. of control RESULTS AND DISCUSSION

Synthesis. R-Benzylphosphonates 5a-5e were synthesized from dialkyl phosphites and benzaldehyde in accordance with Scheme 2. addition of p-toluenesulfonic acid into the imine is highly exothermic and may cause an undesired side reaction. Controlling the reaction temperature near 0 °C during its addition is the most important factor in the synthesis of R-benzylphosphonates. Esters 6a-6c were prepared conveniently from cyanoacetic acid and primary alcohols in the presence of a catalytic amount of anhydrous H2SO4. Amides 6d and 6e were synthesized from ethyl cyanoacetate and the corresponding amine in excellent yields. Intermediate 2-cyano-3,3-dimethylthioacrylate 7 was achieved by treating the corresponding ester (amide) 6a-6e with carbon disulfide and 2 mol of dimethyl sulfate in a one-pot reaction using sodium hydroxide as alkali. The desired products (8a-8x) were then obtained in moderate yields (Scheme 2) by a nucleophilic substitution reaction involving intermediates 7 and R-aminobenzylphosphonate 5a-5e in refluxing ethanol. This reaction is assumed to be initiated by a nucleophilic attack followed by expulsion of SCH3. The Michael-type attack of the R-aminophosphonate at the R,βunsaturated center presumably leads to a transition state in which the orientation of R-aminophosphonate and ester carbonyl is cis due to the presence of an intramolecular hydrogen bonding. The (E)-configuration of the title compounds was established by X-ray single crystal structure analysis of typical 8v, as shown in Figure 1. All compounds were confirmed by elemental analyses and IR, 1H NMR, and 13C NMR spectral data.

Figure 1. Molecular structure of 8v.

Antiviral Activity and Structure-Activity Relationship. To make a judgment of the antiviral potency of the synthesized compounds 8a-8x, the commercially available plant virucide Ningnanmycin (31), perhaps the most successful registered antiplant viral agent available in China, was used as the control. The antiviral bioassay against TMV is assayed by the reported method (9, 30) and the antiviral results of all the compounds against TMV are listed in Table 1. The results showed that most of our designed compounds had moderate antiviral activities at 500 mg/L against TMV in vivo. The title compounds 8a-8x exhibited protection activities of 29.6-56.5% at 500 mg/L. Compounds 8d (R1 is H, R2 is OCH3, and R3 is n-Bu), 8h (R1 is H, R2 is OC2H5, and R3 is n-Bu), 8o (R1 is H, R2 is NH2, and R3 is Et), 8q (R1 is H, R2 is NH2, and R3 is i-Pr), and 8t (R1 is H, R2 is PhCH2NH2, and R3 is Et) have the same protection activity (55.9%, 56.5%, 55.6%, 54.4% and 55.1%, respectively) as that of the standard reference (59.9%). In addition, compounds 8b, 8c, 8e, 8f, 8i, 8j, 8k, 8m, 8n, 8p, 8r, 8s, 8u, 8w, and 8x showed 40.8-48.3% protection activities at 500 mg/L. From the data presented in Table 1, it can be observed that the title compounds 8a-8x possess potential inactivation bioactivities, with values of 64.1%, 47.1%, 45.7%, 92.8%, 92.1%, 71.0%, 37.8%, 84.0%, 74.1%, 46.8%, 51.5%, 55.9%, 80.6%, 70.0%, 60.8%, 63.0%, 5.7%, 9.6%, 70.7%, 51.9%, 50.0%, 51.5%, 11.0%, and 73.4% at 500 µg/ mL, respectively. Among these compounds, 8d and 8e are appreciably more active than the rest, with the inactivation rate of 92.8% and 92.1%, respectively, which are similar to that of Ningnanmycin (99.5%) against TMV at 500 µg/mL. The data also indicate that a change in the substituent might also affect the curative activity of title compounds 8a-8x. Compound 8a (R1 is H, R2 is OCH3, and R3 is Et), 8d, 8e (R1 is 2-F, R2 is OCH3, and R3 is Et), 8f (R1 is H, R2 is OC2H5, and R3 is n-Pr), 8h, 8i (R1 is 2-F, R2 is OC2H5, and R3 is Et), 8j (R1 is H, R2 is OC2H5OC2H5, and R3 is Et), 8m (R1 is H, R2 is OC2H5OC2H5, and R3 is n-Bu), 8n (R1 is 2-F, R2 is OC2H5OC2H5, and R3 is Et), 8o, 8p (R1 is H, R2 is NH2, and R3 is n-Pr), 8t, 8u (R1 is H, R2 is PhCH2NH2, and R3 is n-Pr), and 8v (R1 is H, R2 is PhCH2NH2, and R3 is i-Pr) have curative activities against TMV of up to 56.7%, 60.2%, 58.4%, 50.5%, 55.8%, 50.7%, 55.2%, 51.8%, 52.7%, 54.4%, 53.9%, 55.0%, 53.8%, and 52.9%, respectively, at 500 µg/mL. The other compounds have a relatively lower curative activity than those of 8a, 8d, 8e, 8f, 8h, 8i, 8j, 8m, 8n, 8o, 8p, 8t, 8u, and 8v. Comparison of biological activities among 8a-8x shows functional groups R2 ) OMe, OEt and R3 ) n-Bu to be potentially more active than R2 ) NH2, PHCH2NH2, OC2H5OC2H5 and R3 ) n-Pr, i-Pr, Et.

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Table 1. Protection Effect, Inactivation Effect, and Curative Effect of the New Compounds against TMV in Vivoa

a

agent

concentration (µg/mL)

protection effect (%)

inactivation effect (%)

curative effect (%)

8a 8b 8c 8d 8e 8f 8g 8h 8i 8j 8k 8l 8m 8n 8o 8p 8q 8r 8s 8t 8u 8v 8w 8x Ningnamycin

500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500

29.6* ( 4.9 44.4* ( 4.0 40.8* ( 4.2 55.9 ( 3.8 44.0* ( 3.3 41.6 ( 5.0 37.6 ( 3.0 56.5 ( 3.8 48.3* ( 4.0 41.2* ( 3.2 43.1 ( 2.8 38.4 ( 2.8 44.4* ( 2.9 48.3 ( 4.5 55.6* ( 3.2 45.8* ( 5.2 54.4* ( 4.4 40.2* ( 2.6 42.8* ( 3.2 55.1* ( 5.9 42.4* ( 4.1 39.0* ( 3.5 45.0* ( 2.1 47.7* ( 3.7 59.9* ( 2.5

64.1* ( 1.6 47.1* ( 2.2 45.7* ( 3.0 92.8** ( 1.9 92.1* ( 1.9 71.0* ( 2.8 37.8* ( 4.1 84.0** ( 2.1 74.1* ( 2.8 46.8* ( 3.1 51.5* ( 3.5 55.9* ( 4.9 80.6 ( 1.4 70.0* ( 1.6 60.8* ( 3.7 63.0* ( 2.2 5.7 ( 2.1 9.6 ( 2.4 70.7* ( 2.3 51.9* ( 3.0 50.0* ( 2.9 51.5* ( 2.3 11.0 ( 0.9 73.4** ( 2.8 99.5** ( 2.9

56.7* ( 2.9 37.7* ( 4.6 43.3* ( 2.7 60.2* ( 2.6 58.4* ( 3.4 50.5 ( 4.3 42.8* ( 3.9 55.8* ( 4.2 50.7* ( 3.6 55.2* ( 3.1 43.2 ( 2.7 35.1* ( 4.2 51.8* ( 2.8 52.7* ( 3.9 54.4* ( 3.6 53.9 ( 5.4 28.8 ( 4.8 25.4* ( 3.9 25.4* ( 2.1 55.0* ( 2.9 53.8* ( 4.3 52.9 ( 5.9 28.4 ( 2.5 49.9* ( 2.2 55.8* ( 1.7

All results are expressed as mean ( SD; n ) 3 for all groups; *P < 0.05, **P < 0.01.

Table 2. Antiviral Activities in Vivo (%) of Various Concentrations of Compounds 8d, 8e, 8f, 8h, 8i, 8m, 8n, 8s, and 8x against TMV inactivation effect (%) compd.

500 µg/mL

250 µg/mL

125 µg/mL

62.5 µg/mL

30.7 µg/mL

EC50(µg/mL)

8d 8e 8f 8h 8i 8m 8n 8s 8x Ningnamycin

93.6 93.7 72.0 85.1 75.0 80.2 70.5 71.1 74.9 99.0

70.2 69.7 50.1 65.7 64.1 62.0 48.8 49.0 58.9 80.0

66.9 60.6 47.9 51.6 50.1 53.8 44.2 46.2 48.9 70.0

53.7 52.5 33.1 48.5 39.1 42.8 34.4 35.8 41.2 60.1

37.6 40.8 20.9 42.6 30.7 27.8 20.8 26.9 29.0 42.1

55.5 55.3 159.0 62.6 118.2 87.9 167.3 150.9 127.9 50.9

In addition, as shown in Table 2, compounds 8d, 8e, 8f, 8h, 8i, 8m, 8n, 8s, and 8x were found to display good antiviral activities. These compounds were bioassayed further to investigate their inactivation activities at different concentrations with Ningnanmycin serving as the commercial control. As shown in Table 2, the inactivation effect against TMV of compounds 8d, 8e, 8f, 8h, 8i, 8m, 8n, 8s, and 8x are significant. The EC50 values were 55.5, 55.3, 159.0, 62.6, 118.2, 87.9, 167.3, 150.9, and 127.9 µg/mL, respectively. Among these compounds, 8d and 8e had more potent antiviral activity than the others, being similar to that of Ningnanmycin (EC50 ) 50.9 µg/mL) against TMV. In summary, a series of new cyanoacrylate derivatives containing R-aminophosphonate moieties 8a-8x were designed and synthesized by refluxing a mixture of alkyl 2-cyano-3,3dimethylthioacrylate or cyarylamide and R-aminobenzylphosphonate in ethanol. The in vivo tests indicated that compounds 8d and 8 h exhibited excellent protection effects against TMV and that the curative activity of compound 8d against TMV is much higher than that of Ningnanmycin, while a very similar inactivation bioactivity level of compounds 8d and 8e and Ningnanmycin against TMV was observed. Therefore, the present work demonstrates that the antiviral activity of cyanoacrylate derivatives was significantly improved via the introduction of the R-aminophosphonates moiety. Effects of steric, hydrophobic, electrostatic, and electrostatic parameters

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Received for review February 9, 2008. Revised manuscript received March 22, 2008. Accepted April 10, 2008. We thank the National Key Program for Basic Research (No. 2003CB114404), the Cultivation Fund of the Key Scientific and Technical Innovation Project, the Ministry of Education of China (No. 705039), the National Basic Research Priorities Program (No. 2005CCA01500), and the National Natural Science Foundation of China (No. 20672024) for financial support.

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